“神舟7号”上天后，新一代长征二号F火箭的推进剂将换成无毒、无污染的全液氧煤油推进剂．现有的四氧化二氮和偏二甲肼（C2H8N2）推进剂有一定的腐蚀性和毒性，推力小，成本高，即将面临淘汰．偏二甲肼燃烧的化学方程式为：C2H8N2+2N2O4═3N2+4H2O+2CO2．据以上内容分析，以下说法正确的是（ ）
A．偏二肼和液氧都具有可燃性
B．偏二肼燃烧时，反应物和生成物的化学计量数总和为11
C．四氧化二氮和氮气中氮元素的化合价都是+4
D．推进剂的改进有助于改善环境，保护环境
答案：D
(2013·高考新课标全国卷Ⅰ)I went to a group activity，“Sensitivity Sunday”，which was to make us more ____ the problems faced by disabled people.We were asked to“___a disability” for several hours one Sunday.Some members，____，chose to use wheelchairs.Others wore soundblocking earplugs (耳塞) or blindfolds (眼罩)． Just sitting in the wheelchair was a____experience.I had never considered before how____it would be to use one.As soon as I sat down，my____made the chair begin to roll.Its wheels were not___.Then I wondered where to put my____.It took me quite a while to get the metal footrest into____.I took my first uneasy look at what was to be my only means of____for several hours.For disabled people，“adopting a wheelchair”is not a temporary(临时的)___. I tried to find a____position and thought it might be restful，____kind of nice，to be___around for a while.Looking around，I____I would have to handle the thing myself! My hands started to ache as I____the heavy metal wheels.I came to know that controlling the____of the wheelchair was not going to be a(n)____task. My wheelchair experiment was soon____.It made a deep impression on me.A few hours of “disability” gave me only a taste of the____，both physical and mental，that disabled people must overcome.
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